Field of the Invention
The present invention relates to heat-resistant alloy steels having improved resistance to oxidation, and corrosion at elevated temperatures as well as to carburization, nitrization and sulphurdization.
In recent years, metal materials used in equipments related to energy industries, such as coal liquefaction and gassification, MHD, distillation of oil shale and tar sand and petroleum processing, have been encountering more and more severe high temperature service environments.
The problem common to these applications is that the metal materials used in these equipments are severely attacked by oxidation and corrosion by the high concentration of S, Cl Na, K, P, C or their compounds, and in some cases, by the elevated temperature service environments containing molten salts, such as sulfates and carbonates. In these service environments, conventional stainless steels and heat resistant steels have been found to show only poor resistance to the oxidation and corrosion at elevated temperatures, and heat-resistant materials with increased resistance to these attacks have been in urgent demand. Also in the fields of furnaces for heating metal and refractory materials, furnace damages due to oxidation, carburization, nitrization, etc. in various high temperature service environments have long been a serious problem to be solved. Furthermore, from the viewpoint of energy conservation, there is a recent tendency to eliminate water cooling, and to replace the part with heat-resistant materials. In this respect, the further improvement of heat resistance of these materials have been in ever increasing demand.
For the purpose of increasing the resistance of heat-resistant steel materials to oxidation and corrosion at elevated temperatures in general, it is effective to increase the contents of Cr, Si and Al in the steel materials. However, excessive contents of these elements tend to deteriorate the fluidity of molten metal and the toughness and ductility required in their hot working and cold working, resulting in poor productivity, workability and service performance. Therefore, the addition of these elements are limited.
In order to solve the above contradictions, various proposals have been made, such as applying Al coatings on austenite stainless steels, Ni-base or Co-base heat-resistant alloys, or diffuse the Al coatings into the base metals, as disclosed in Japanese Laid-Open Patent Specification No. Sho 55-97460, Japanese Patent Publication No. Sho 56-505, "Journal of Metals" G. W. Goward, Oct. 1970, Vol. 31 and Nippon Gakujutsu Shinko-kai No. 123 Committee Report Vol. 19, No. 2, page 233, July 1978.
In these prior arts, the proposals for Ni-base alloys or Co-base alloys are of less practical use because of their very high material costs. On the other hand, some of austenite stainless steels and heat-resistant cast steels coated with Al or surface-diffused with Al are used in partial applications. However, these Al-coated or Al-diffused steel materials have encountered the following problems.
The Al coated or diffused on the steel surface, when used at a service temperature higher than the melting point of Al, mutually diffuses with the steel surface, forming distinctive two layers: an alloy layer with higher Al content and a diffused layer with lower Al content beneath the former, and between these two layers of different Al contents a lot of voids are formed in line due to the difference in the metallic diffusion rate, so that the high Al alloy layer spalls off to expose the low-Al diffused layer to the high temperature environments. Although the low-Al diffused layer thus left has better resistance to oxidation and corrosion at elevated temperatures than the metal base without the Al coating, the Al content in the layer gradually decreases as Al diffuses into the matrix of the base steel, and eventually the effect of the Al coating is nullified. In some cases, the low-Al diffused layer may spall off at the boundary between the diffused layer and the base steel. For these reasons, the Al coating or Al diffusion on austenite stainless steels or heat-resistant cast steels are not reliable, because they cannot maintain their desired effects for a long period of service time.
Meanwhile an art of applying paint coating containing powdered Al on Al-containing stainless steels is disclosed in Japanese Laid-Open Patent Specification No. Sho 54-123534. However, this prior art is to form a stable Al.sub.2 O.sub.3 film by firing on the steel which is difficult to form a uniform Al.sub.2 O.sub.3 film thereon because of internal oxides etc. caused by hot working. Thus according to this prior art, Al in the paint coating is oxidized during a heating process or an initial stage of soaking process where the Al in the steel does not fully diffuse, so as to temporarily prevent intervention of oxygen to the steel surface, and hence prevent further internal oxidation of the steel, and at the stage when all Al in the paint coating is oxidized the Al in the steel is allowed to fully diffuse so that a selective oxidation of Al takes place on the surface of the steel, forming a uniform Al.sub.2 O.sub.3 film thereon.
However, this prior art suggests nothing of the formation of alloy layer, and particularly the technical problem encountered by the prior art of Japanese Laid-Open Patent Specification No. Sho 55-97460 and Japanese Patent Publication No. Sho 56-505, etc., and cannot solve the technical problem, because powdered Al dispersed in the paint coating, etc. cannot form an Al alloyed layer.